Many applications of laser radiation require long duration and/or low peak power pulses. For example, a transform-limited pulse of duration about 500 ns is required to achieve a single-shot velocity resolution of about 1 ms−1 in an eye-safe coherent laser radar system. In another example, a macro-pulse envelope of duration 1-3 μs is required for artificial guide-star laser sources in multi-conjugate adaptive optics on extremely large optical telescopes. Low peak power pulses are also required in biological/medical applications to prevent damage due to ionisation that might be caused by high peak power within the pulse.
Q-switched lasers are often used to produce pulses for applications requiring energetic pulses. The peak power in a laser pulse can be minimised for a given pulse energy by ensuring that the pulse consists of a single optical mode, thereby preventing interference between lasing modes that would produce high peak power spikes within the pulse, and by increasing the duration of the pulse. However, while there are established techniques for producing single-mode pulses from Q-switched lasers, these lasers typically produce pulses that have durations less than about 50.
Existing techniques for ensuring single-mode content of a pulse include the use of etalons within the resonator, injection seeding and servo control of the resonator length. Injection seeding of the Q-switched laser by the single frequency output of a continuous wave master laser is the preferred technique in a number of applications including remote sensing, as the output of the master laser can also then be used as the reference oscillator in the detection system. Also, it does not require additional components within the Q-switched laser.
There are a number of techniques by which the duration of a Q-switched pulse can be increased, or “stretched”. These include: (1) increasing the optical length of the resonator; (2) increasing the output coupling of the resonator or (3) adjusting and “throttling” the Q-switch so as to increase dynamically the output coupling of the laser resonator.
With regard to the first technique referred to above, it is clear that the option of increasing the resonator length will involve significant changes to the resonator configuration which may not be possible given the physical requirements and environment for the system. As would be apparent to those skilled in the art, each time the resonator length is changed it will be necessary to realign the laser making this approach unsuitable for those circumstances where flexibility is required in terms of defining the duration and energy of the laser pulse produced by the system.
Turning now to the second technique, this approach involves extending the duration of a pulse by increasing the output coupling of the laser. This effect can be achieved by either increasing the transmission of the out-coupling mirror or alternatively by adjusting the retardation provided by an intra-resonator wave-plate if the output coupling is achieved via an intra-resonator polarizer. However, increasing the transmission of the out-coupling mirror requires replacement of the mirror once again necessitating an overall realignment of the resonator. A tunable mirror in which the spacing between several mirrors is adjusted could possibly be used as the out-coupling mirror in order to provide dynamic control of the pulse width by changing the transmission characteristics of the mirror. However, this approach is complicated and the output coupling is not linearly related to the spacing of the mirrors.
Changing the output coupling and thereby the resultant pulse width via polarization control requires either mechanical adjustment of an intra-resonator wave plate or electrical adjustment of an intra-resonator Pockels Cell. The latter approach has only been demonstrated for standing-wave lasers and in cavity-dumped ring lasers that are optimised for the production of high peak power pulses and are not suitable for those applications where lower peak power Q-switched pulses are required.
One example of this approach includes the stretched-pulse injection-seeded Q-switched standing-wave laser described in U.S. Pat. No. 5,237,331 where in this system a Pockels cell and a polarizer are used to Q-switch a slave oscillator and the out-coupling is set by the orientation of an intra-resonator quarter-wave retardation plate. Whilst this laser system is able to produce a stretched pulse efficiently, it is a point design resulting in a fixed pulse width that cannot be adjusted in real-time. In addition, to minimize energy losses due to Fresnel reflections, it would be expected that the gain medium would need to include anti-reflection coated entrance and exit faces which will limit the maximum pulse energy as the anti-reflection coatings may be optically damaged at high pulse energies. Finally, standing-wave lasers in general have the disadvantage that they are subject to spatial hole burning which complicates the production of a single-mode output making these lasers unsuitable for coherent remote sensing applications.
Injection-seeded Q-switched travelling-wave or ring lasers for the production of narrow-band pulses have been described in U.S. Pat. No. 5,305,334 and US Patent Application No. 2002/0185701. However, both of the systems described in these documents use transmissive out-coupling mirrors and thus efficient dynamic control of the pulse duration cannot be achieved as pulse energy would be discarded from the resonator if the Pockels cell Q-switch was throttled. Furthermore, the output pulses would have a mixed polarization state, which is unsuitable for remote sensing applications.
The third technique to increase the pulse width is by the use of Q-switch throttling (see for example W. E. Schmid, IEEE J. Quantum. Electronics Vol. QE-16(7) pp. 790-4 (1980)) which in principle should allow for the dynamic variation of both pulse width and energy. This approach has only been demonstrated using standing-wave resonators with an out-coupling mirror which, as described previously, results in a system with low energy efficiency.
In summary, there have been a number of techniques for producing long duration Q-switched pulses that have been demonstrated in the prior art. However, these approaches all suffer from one or more of the following disadvantages: inefficiency, spatial-hole burning, dynamic control not possible, single point operation, inability to be used with Brewster-angled gain media, and mixed polarization state output.
It is an object of the present invention to provide a system for producing a laser light pulse capable of efficiently generating a pulse of variable duration and peak power.
It is a further object of the present invention to provide a system capable of producing a laser light pulse with a substantially plane polarization state.